Are you using the default AMP6 at the moment?
Perhaps AMP3 would be a good solution if you're already happy with the lows and mids as you say?
Your chances of getting a cheap one second hand are quite high I guess too.
And it matches, connection-wise, with the CB12s cable you're using already
It would seem fair to say that the majority of people on here seem to think AMP8 is the best one; however, that would require using an adapter or getting a new cable. And I'm not sure, but I have a vague feeling that AMP3 could be more suited to the requirements you've outlined.
I recently switched over to AMP3 on my DX150 (from AMP7).
Haven't listened properly to it yet, but I think it's going to sound pretty great
Food for thought anyway, hope this helps